The best replacement for solr is lucene.

The best alternatives to solr are rated as following:
★★★★★  lucene
★★★★★  sphinx
★★★  elasticsearch
★★  endeca
★  mongodb
★  fast
★  hadoop
★  cassandra
★  google search appliance
★  search
★  filter query
★  solrcloud
  nutch
  gsa
  thinking sphinx
  google search
  xapian
  hibernate search
  splunk
  verity

Your alternative for solr is not listed? Suggest!
Speaking about solr, one probably shouldn't forget about tomcat.

Other search terms related to solr and their ratings are:
★★★★★ tomcat
★★★ cassandra
★★★ zookeeper
★★ hbase
You have in mind a companion for solr and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of solr in all years. Click on graph for more.
solr
lucene
sphinx